It is known to provide common amplification in television transmitters wherein the visual and aural carriers are commonly amplified. The United States patent to Ta et al. 5,198,904 noted that a problem resulting from such common amplification is aural carrier distortion. The distortion occurs in both the phase and amplitude domains of the aural carrier.
The patent to Ta proposes to correct the unwanted distortion of the aural carrier by predistorting phase and amplitude components of the aural carrier so as to be directly opposite to the unwanted distortions caused by the common amplification in a television transmitter. To this end, Ta samples the baseband video signal prior to the signal being used to modulate a visual IF carrier signal. The sampled baseband video signal is delayed by a video delay to compensate for the delays that are introduced in an IF vision modulator to which the baseband video signal is also supplied for modulating a visual carrier signal. The delayed baseband video signal is then applied to a complementary nonlinear amplifier which provides a phase correction signal and an amplitude correction signal. These correction signals are supplied to an amplitude and phase modulator which also receives the aural carrier. The aural carrier is modulated by the amplitude and phase correction signals to provide a modified aural signal which is summed with the output of the IF vision modulator.
It has been found that the sampled baseband video signal in the Ta et al. patent has a relatively flat amplitude response over the baseband video spectrum. However, the cross-modulation on the aural carrier has two different sideband levels. The high level is created by the double sideband region of the visual spectrum, that is .+-.0.75 MHz about the visual carrier frequency. The half amplitude levels extending to .+-.4.2 MHz about the aural carrier are due to the single sideband region (0.75-4.2 MHz) of the visual spectrum. Therefore, when modulating the aural carrier with frequency and amplitude correction signals based on Ta's flat spectral response (of the baseband signal) an incomplete cancellation of the cross-modulation will result. That is, if the .+-.0.75 MHz region is eliminated, then the single sideband regions will be over corrected. The over-correction can result in line rate sidebands being created about the aural carrier which falls back into the visual passband. Because the phase of these sidebands is not controlled, they add and subtract from the wanted visual line rate sidebands causing a change to the visual spectrum. The net result is that black to white transitions and sync pulse edges have a fuzzy appearance.
Another problem noted in the Ta et al. patent is the cost and complexity of a video delay circuit which serves to delay the baseband video signal prior to developing the correction signals. This video delay serves to compensate for the delays introduced in the IF vision modulator and filter where the baseband video signal modulates the visual IF carrier signal. Such delay circuits are expensive and, consequently, it is desirable to design circuitry to minimize or eliminate the use of such circuits.